Motohide Matsui

Refinement of RE211 particles in melt-textured RE-Ba-Cu-O bulk superconductors

The formation mechanism of fine Gd211 particles was studied by observing the size and morphology of Gd211 in the Gd–Ba–Cu–O melt-quenched from 1100 °C with various Gd211 particle sizes. The size and distribution of Gd211 particles at the partial melting stage depended on the size of initially added Gd211 particles. The precursor with fine Gd211 starting powder formed fine needle-shaped Gd211 particles after melting. The Gd211 refinement is ascribable to the increased number of Gd211 nucleation sites and growth inhibition by Pt addition. We also studied the refinement of RE211 (Nd422) particles in other RE–Ba–Cu–O (RE: Y, Dy, Eu, Sm, Nd) systems. In the cases of Y-, Dy- and Nd-systems, the size of RE211 (Nd422) particles could be reduced by decreasing the size of RE211 (Nd422) starting powders. In contrast, the employment of fine RE211 starting powder was not so effective for the size reduction of RE211 particles in the Eu- and Sm-systems.

Effect of Ag2O addition on the trapped fields and mechanical properties of Nd–Ba–Cu–O bulk superconductors

We investigated the effect of Ag2O addition on the microstructures, the superconducting properties, the field-trapping capabilities and the mechanical properties of Nd–Ba–Cu–O (Nd123:Nd422 = 10:2) bulk superconductors. The samples were added with 10 wt% and 20 wt% Ag2O and grown in 1% O2–Ar atmosphere. The superconducting properties and the field-trapping capabilities were not affected by Ag2O content; however, the mechanical properties were greatly improved with increasing Ag2O content. The sample with 20 wt% Ag2O exhibited the Weibull coefficient exceeding 13.

Single-grain growth, microstructure and superconducting properties of (Sm, Eu)123 superconductors with BaCuO2−x addition

High quality (Sm0.5, Eu0.5)Ba2Cu3O7−δ single domains were grown in air with BaCuO2−x addition. It was found that a RE–Ba substitution (RE: Sm and Eu) could be effectively suppressed by a small addition of BaCuO2−x. The 94 K superconductivity with sharp transition was obtained in the composition range of 0.16 ~ 0.33 BaCuO2−x and (Sm0.5, Eu0.5)Ba2Cu3O7−δ in a molar ratio. At 77 K, a surface field of 0.8 T was trapped in a single-domain pellet of 3.2 cm diameter. A further addition of BaCuO2−x deteriorated the microstructure and degraded the superconducting performance.

Mechanical properties of Nd?Ba?Cu?O bulk superconductors

We investigated the effects of Nd422 and Ag particles on the mechanical properties in Nd–Ba–Cu–O bulk superconductors. Both Nd422 and Ag particles were effective in decreasing the amount of microcracks running along the c direction. In the case of Nd422, however, excessive Nd422 addition enhanced the crack propagation, resulting in the degradation of mechanical strength. In the case of Ag addition, the beneficial effect of its ductile mechanical property was not observed. This was due to a relatively large size of Ag particles and low interfacial strength between Ag and Nd123 matrix. It was remarkable that the Weibull coefficient of the sample with Ag2O addition exceeded 13, which is reliable enough for practical engineering applications.

Fabrication of large-grain Nd–Ba–Cu–O superconductor

Abstract We fabricated a c -axis oriented Nd–Ba–Cu–O bulk superconductor with 10 wt.% Ag 2 O addition using the top-seeded melt-growth technique in 1% O 2 –Ar atmosphere. The size of the sample was 30 mm in diameter and 10 mm in thickness. We measured the field dependence of the trapped field at various temperatures. The peak effect of the trapped field could be observed, showing that the field-induced pinning is active even in large dimensions. The peak value of the trapped field reached 1.1 T at liquid nitrogen temperature. A high trapped field value of 3.75 T was recorded at 50 K.

Effect of Nd422 addition on the mechanical properties of Nd–Ba–Cu–O bulk superconductors

Abstract We studied the mechanical properties of Ag 2 O-doped Nd–Ba–Cu–O bulk superconductors with different initial compositions grown in 1%O 2 –Ar atmosphere. The average tensile strength increased with increasing Nd 4 Ba 2 Cu 2 O 10 (Nd422) content. The sample with the composition of Nd123:Nd422=10:2.5 exhibited the average strength of 56.8 MPa. Furthermore, the Weibull coefficient of this sample exceeded 8. Such improvement is ascribed to the fact that Nd422 particles can suppress the propagation of microcracks perpendicular to a – b planes.

Effect of silver addition on the field trapping properties of Gd–Ba–Cu–O bulk superconductors

Abstract The effect of Ag addition on the field trapping properties of Gd–Ba–Cu–O bulk samples has been studied. Gd–Ba–Cu–O bulk superconductors 32 mm in diameter were melt textured by varying the amount of Ag 2 O in the range of 0–30 wt.%. The trapped magnetic field of Ag-free bulk sample was 1.3 T at 77 K. This value could be improved to 1.8–2.0 T with Ag addition of 10–20 wt.%. A further increase in Ag 2 O content degraded the field trapping ability. Ag-free bulk Gd–Ba–Cu–O exhibited the trapped field of 3.0 T at 65 K, however, the bulk was fractured at lower temperature. On the other hand, Ag-added bulk Gd–Ba–Cu–O exhibited the high trapped field value of 6.7 T at 55 K. These results indicate that the mechanical strength of the bulk Gd–Ba–Cu–O could be improved with Ag addition.

Effect of ZrO2 and BaZrO3 addition on the microstructure and superconducting properties of melt processed NdBa2Cu3O7-δ

Abstract We investigated the microstructure and superconducting properties of the Nd–Ba–Cu–O with 0–1.0 wt.% ZrO 2 and 0–2.24 wt.% BaZrO 3 synthesized by the oxygen-controlled melt-growth process in 1% O 2 –Ar atmosphere. The microstructure of Zr-added NdBCO was not homogeneous, and one could observe the segregation of Nd422 particles, the residual liquid phase including the agglomeration of BaZrO 3 particles and the Nd422-free regions. Such inhomogeneity is caused by the strong influence of BaZrO 3 on the solid/liquid interfacial energy during the melt growth.

Effects of Pt and CeO2 addition on the growth of Nd4Ba2Cu2O10 particles

We studied the effects of Pt and CeO2 addition on the growth of Nd4Ba2Cu2O10 particles using three different starting materials: Nd123-Nd422, Nd422-BaO2-CuO and Nd2O3-BaO2-CuO, all having the same nominal composition of Nd:Ba:Cu = 1.8:2.4:3.4. It was found that both Pt and CeO2 had beneficial effects on the size reduction of Nd422, although CeO2 addition was more effective. Furthermore, it was also clarified that combined addition of Pt and CeO2 was the most effective in refining Nd422 particles and thus increasing the JC values at 77 K.

Effect of cooling rate on the trapped-fields of Nd–Ba–Cu–O bulk superconductors

Abstract We fabricated Ag2O-doped single-domain Nd–Ba–Cu–O bulk superconductors in 1% O2–Ar atmosphere with using different cooling rates during the crystal growth. The peak effect could not be observed in the field dependence of the maximum trapped-field for the sample fabricated with slower cooling rates. We could obtain higher trapped field values in all the fields in the sample fabricated with the faster cooling rate. It was found that processing conditions strongly affected the performance of Nd–Ba–Cu–O bulk superconductors.